Investigation of somatic and gonadal growth of female rainbow trout (Oncorhynchus mykiss) reared in cage in the Caspian Sea

The aim of the present study is to investigate the growth performance and sexual maturation process of female rainbow trout reared in a cage in the Caspian Sea (Mazandaran- Nowshahr). Rainbow trout with an average weight of 122± 5 g were stocked into a cage located in Caspian Sea. Fish were fed daily and random sampling was done monthly (January 2015) to May 2016. Weight and length of thirty fish were measured; their gonad and liver were weighted; total length, total weight, liver and gonad weight, condition factor, GSI and HSI and oocyte diameter were calculated; their blood samples were collected and the levels of 17 beta estradiol, testosterone, progesterone and the vitellogenin were measured; also gonad tissue samples were prepared and examined using a light microscope. The results showed optimal somatic and gonadal growth of fish caged in the Caspian Sea. Sex hormones and vitellogenin levels increased significantly (P<0.01) during the study period. The levels of 17 beta estradiol, testosterone and progesterone in January were 0.56± 0.05 ngml-1, 0.50± 0.05 ngml-1and 0.14± 0.01 ngml-1 respectively and reach to 1.63± 0.13 ngml-1, 0.80± 0.01 ngml-1 and 0.28± 0.03 ngml-1 in May, respectively. Vitellogenin levels increased from 382.40± 55.26 ngml-1 in January to 651.40± 86.34 ngml-1 in May. The gonads developed to stage 4 and the oocyte diameter increased significantly (P<0.01) during the study period. The findings of present study show that female rainbow trout are able to have appropriate somatic growth in the brackish water of the Caspian Sea like fresh water, and the natural process of sexual maturation won’t be disturbed.